A push-pull MIG welding system generally includes a wire feeder, and a welding torch connected to the wire feeder. Electrode welding wire (typically aluminum), shielding gas, welding current, and/or coolant are transferred inside the welding torch. The welding torch includes a handle connected at a rear end to a cable assembly and at a front end to a gooseneck which terminates in a contact tip assembly. The wire feeder includes a pushing drive roll mechanism that pushes the welding wire through the cable assembly to the handle. A pulling drive roll mechanism in the torch handle pulls the welding wire from the cable assembly and sends it forward through the gooseneck to the contact tip assembly. A tubular jump liner typically extends from the torch handle to the contact tip assembly to protect the welding wire as it travels through the gooseneck.
It is known in the art relating to push-pull MIG welding torches that the pulling drive roll mechanism in the handle of the torch is open to free space inside the handle. Thus, the inner diameter (ID) of the jump liner, inside which the electrode wire transits, is also open to the free space at this end. In order to deliver shielding gas through the gooseneck to the contact tip assembly, sealing is required between the jump liner and the gooseneck along the whole length of the jump liner. Current push-pull MIG torch designs provide adequate sealing between the jump liner and the gooseneck or body block inside the handle, but do not provide sealing between the jump liner and the retaining head or contact tip at the front end.
In order to reduce shaving between the jump liner and aluminum welding wire, most jump liners are made of plastic materials such as PTFE, nylon, and the like. One characteristic of plastic jump liners is the large dimensional variation based on factors such as temperature, humidity, and manufacturing. Therefore, significant gaps are required between a plastic jump liner and the metal parts of the welding torch to allow for expansion and contraction of the jump liner. These gaps, however, provide undesirable leakage channels for shielding gas. If a plastic jump liner is cut too short, the gap between the jump liner and the retaining head of the contact tip assembly allows for back flow of pressurized shielding gas into the jump liner and escape at the rear end of the jump liner (the end towards the pulling drive rolls inside the torch handle). On the other hand, if the jump liner is cut too long, the resiliency of the jump liner causes the front end of the jump liner to mushroom or even close. This effect is more pronounced when the front end of the torch is hot, which softens the jump liner. In any event, this results in a closing of the internal space between the jump liner and the electrode wire, eventually jamming the wire.